Curium

History

Curium was chemically identified at the Metallurgical Laboratory (now Argonne National Laboratory) at the University of Chicago. It was the third transuranium element discovered, though it is the fourth in the series; the lighter element americium was still unknown at the time. The sample was prepared by coating plutonium nitrate solution on a platinum foil, evaporating it, and converting the residue into plutonium(IV) oxide by annealing.

After cyclotron irradiation of the oxide, the coating was dissolved with nitric acid and precipitated as the hydroxide using concentrated aqueous ammonia. The residue was dissolved in perchloric acid, and ion exchange separated a curium isotope. The separation of curium and americium was so difficult that the Berkeley group initially called them pandemonium (from Greek for all demons or hell) and delirium (from Latin for madness).

In July–August 1944, 242Cm was made by bombarding 239Pu with alpha particles, releasing a neutron. It was identified by the characteristic energy of alpha particles emitted during its decay to 238Pu; its half-life was first measured as 150 days and later corrected to 162.8 days. Another isotope, 240Cm, was produced in March 1945 via a similar reaction, with a half-life initially determined as 26.8 days and later revised to 30.4 days. The discovery was part of the Manhattan Project, so results were confidential and declassified only in 1945.

Seaborg leaked the synthesis of elements 95 and 96 on the U.S. radio show Quiz Kids five days before the official presentation at an American Chemical Society meeting on November 11, 1945. The discovery and its compounds were later patented listing only Seaborg as the inventor. The element was named after Marie Curie and her husband Pierre Curie, following the example of gadolinium, named after Johan Gadolin.

Characteristics

Curium is a hard, dense, silvery-white metal with physical and chemical properties resembling gadolinium. Its melting point of 1344 °C is higher than that of neptunium (637 °C), plutonium (639 °C), and americium (1176 °C), while gadolinium melts at 1312 °C. Curium boils at 3556 °C and has a density of 13.52 g/cm³, lighter than neptunium and plutonium but heavier than most other metals. At ambient conditions, the α-Cm crystalline form is more stable, with hexagonal symmetry and a double-hexagonal close packing structure isotypic with α-lanthanum.

Under pressure above 23 GPa, α-Cm transforms to β-Cm with face-centered cubic symmetry; further compression to 43 GPa yields γ-Cm, an orthorhombic structure similar to α-uranium. Curium exhibits peculiar magnetic properties: α-Cm becomes antiferromagnetic upon cooling to 65–52 K, while β-Cm shows a ferrimagnetic transition at about 205 K. Curium pnictides such as 244CmN and 244CmAs undergo ferromagnetic transitions upon cooling. Electrical resistivity of curium increases with temperature, roughly doubling between 4 and 60 K, then remains nearly constant up to room temperature; resistivity also increases over time due to self-damage from alpha decay.

Synthesis

Curium is produced in small amounts in nuclear reactors, with only kilograms of 242Cm and 244Cm accumulated; heavier isotopes exist in grams or milligrams. The high price reflects this scarcity: about 2000 USD/g for 242Cm and 170 USD/g for 244Cm. In reactors, curium forms from 238U through a series of neutron captures and beta decays, eventually yielding americium and then curium.

For research, plutonium from spent nuclear fuel is irradiated with a higher neutron flux to produce 244Cm. Curium-244 alpha decays to 240Pu but also absorbs neutrons, yielding small amounts of heavier isotopes. Isotopes 247Cm and 248Cm are popular for research due to their long half-lives, though 247Cm production is low because it readily fissions with thermal neutrons.

Synthesis of 250Cm is unlikely because the intermediate 249Cm has a short half-life (64 minutes) and beta decays to 249Bk. The most efficient way to obtain 248Cm is via alpha decay of 252Cf, which yields about 35–50 mg per year with 97% isotopic purity. Another isotope, 245Cm, can be obtained from alpha decay of 249Cf, itself produced from 249Bk.

Compounds and reactions

Curium readily reacts with oxygen, forming mostly Cm2O3 and CmO2, though the divalent oxide CmO is also known. Black CmO2 can be obtained by burning curium oxalate, nitrate, or hydroxide in pure oxygen; heating it to 600–650 °C in vacuum transforms it into whitish Cm2O3, which can also be produced by reducing CmO2 with molecular hydrogen. Ternary oxides of the type M(II)CmO3 exist, where M is a divalent metal like barium.

Thermal oxidation of trace curium hydride yields volatile CmO2 and CmO3, one of two known examples of the +6 state for curium; a species tentatively characterized as CmO4 (with curium in the +8 state) has been reported, but new experiments indicate it does not exist. Colorless curium(III) fluoride (CmF3) forms by adding fluoride ions to curium(III) solutions, while brown curium(IV) fluoride (CmF4) requires reaction of CmF3 with molecular fluorine. Ternary fluorides of the form A7Cm6F31 (A = alkali metal) are known. Colorless curium(III) chloride (CmCl3) is made by reacting curium hydroxide with anhydrous hydrogen chloride; it can be converted to curium(III) bromide or curium(III) iodide by reaction with the corresponding ammonium halide at 400–450 °C. Vapor phase hydrolysis of CmCl3 yields curium oxychloride.

Sulfides, selenides, and tellurides are obtained by treating curium with the respective element in vacuum at high temperature. Curium pnictides (CmX for nitrogen, phosphorus, arsenic, antimony) are prepared by reacting curium(III) hydride or metallic curium with these elements. A stable curocene complex (η8-C8H8)2Cm is predicted by molecular orbital theory but has not been experimentally reported.

Did You Know?

• Curium is one of the most radioactive isolable elements; 242Cm and 244Cm emit alpha particles with energy 6 MeV and produce 120 W/g and 3 W/g of heat, respectively.
• If consumed, only 0.05% of curium is absorbed into the blood; from there, about 45% goes to the liver, 45% to the bones, and the remaining 10% is excreted.
• The biological half-life of curium is about 20 years in the liver and 50 years in the bones.